As shown in FIG. 1, a typical skateboard 10 is composed of a long elliptical plate 11 and two pairs of wheels 12A and 12B fixed to the bottom of the plate 11. The pairs of wheels 12A and 12B each have two rollers.
In general, a rider on the skateboard 10 propels it by pushing the ground with one foot when traveling on a flatland, and inclines the skateboard 10 to the left or right with respect to the traveling direction with feet to turn while traveling.
However, since the rider have to push the ground with foot every time to propel the board, he/she loses an interest in driving, and a safety accident may occurs when the rider is unfamiliar with steering. Further, it is difficult for unskilled riders to try turning in traveling and there is a large possibility that a rider falls down when the skateboard is inclined more than a predetermined degree to the sides.
Accordingly, Korean Patent Registration No. 0394848 discloses a skateboard that can be propelled only by twist of the rider's body and turn with a small radius of curvature in traveling.
That is, as show in FIG. 2, the skateboard 20 is composed of two boards, that is, a front board and a rear board 21A and 21B, directional casters 22A and 22B fixed at an angle to the bottom of the boards 21A and 21B, a torsion pipe 23 connecting the boards 21A and 22B and having an elastic built-in member for restoration against torsion and bending, and fastening members 24 fixing the pipe 23.
Further, the torsion pipe 23, as shown in FIGS. 3 and 4, two pipes, front and rear pipes 25A and 25B are connected with each other by a bearing 26, such that they can idle with respect to each other. A spring 27a and a locker 27b are provided at an end of an inner pipe 25C disposed inside the pipe 25B. A leaf spring 28 is provided inside the inner pipe 25C and fixed in a holder 29 by a screw 29a. 
Therefore, elastic deformation occurs by relative torsional motion between the front and rear pipes while the boards travel, and the torsion pipe 23 restores when the torsional external force is removed. Accordingly, the front board 21A and the rear board 21B make wave-shaped motion while elastically rolling in the opposite directions. The skateboard can be moved forward by this motion of the front board 21A and the rear board 21B.
However, the skateboard has a problem that workability is deteriorated, because it needs many parts, including a plurality of pipes for connecting both boards, a bearing disposed between the pipe, and a locker and a spring for preventing idling.
Further, a rider has to twist his/her body left and right on the board to generate propulsive force. Accordingly, large external torsion force is exerted in the joints of the parts, such that they are broken by cracks. Further, a rider has a difficult in balancing both boards due to decrepitude of the leaf spring by use for a long period of time, which makes it to ride the skateboard. Since the propulsive force is generated by the rolling motion of both boards, it is difficult to generate propulsive for that can satisfies the rider.
On the other hand, the applicant(s) of the present invention had repeatedly studied on the basis of the relate arts described above, and as disclosed in Korean Paten Registration No. 0576306, has been granted a patent about a “skateboard”, which includes a front board and a rear board equipped with a caster, a center connecting portion that combine the boards, a support means inserted in a hole formed in the center connecting portion, hinge means and fastening means fixed to the upper and lower portion of the center connecting portions. Further, patentability has been recognized for a “skate board equipped with a brake system”, which can be easily braked by a brake system disposed at the center between both boards, as disclosed in Korean Patent Publication No. 2008-0025907.
In those skateboards have the advantage of allowing a rider to easily keep his/her balance on both boards and smoothly travel at a sharp curve. Further, a rider can easily carry the skateboards and easily brake them traveling. However, these skateboard still have a problem that riders cannot help but moving their bodies to generate propulsive force from rolling motion of both board.